Network Topology

Criteria

1. Cost of building network and of adding new nodes.
2. Speed of getting message from some node A to other B.
3. Reliability of network - if some links or nodes down, does any of it still work?

• Network topology

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Fully-connected Network

Only cost-practical in local network.

1. Cost - Couldn't be higher.
2. Speed - Couldn't be better.
3. Reliability - Couldn't be better unless had duplicate links.

Example: Some military local networks.

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Partially-connected Network

The only practical large-scale topology. Grows as traffic grows. e.g. Internet. Once only 1 trans-Atlantic link. As traffic grows, more links added. Any site can be added by just 1 connection to any existing site. If traffic increases may add 2 connections to the rest of the Net, and so on. Variable-resolution.

1. Cost - Trade off with speed and reliability.
2. Speed - Trade off with cost. If too slow, add more links.
3. Reliability - Trade off with cost. If not reliable enough, add more links. Variable-resolution.

Example: Internet.

See growth of Internet 1969-72.

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Linear or Bus network

All machines attached to one cable.
This is a broadcast network: All machines see all comms on the cable.

Only practical for local network.
Normally will have a HQ router - the LAN's gateway to the outside world.

1. Cost - Very cheap.
2. Speed - Excellent.
3. Reliability - If cable fails, whole network fails.

Example: Ethernet LAN.

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Ring Network

Only practical for local network.

1. Cost - Fairly low.
2. Speed - OK.
3. Reliability - Partitioned easily, but comms inside partitions still work.

Example: Common local network topology.

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Star Network

Only practical for local network.

1. Cost - Very cheap.
2. Speed - Excellent in terms of distance, but could be traffic jam at HQ.
3. Reliability - Dangerous. If HQ down, whole network fails.

Example: Common local network topology.

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Hierarchical Network

Could be built globally but no one would use it. Only practical for local, in-house, network.

1. Cost - Very cheap.
2. Speed - Possible traffic jams at HQ. But perhaps no more so than partially-connected with 1 trans-Atlantic link. Much traffic stays within each side.
3. Reliability - Network partitioned easily, but partitions still work.

Example: High-level organisation of multiple LANs can look a bit like this.